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[GYMusic]

AKG C-414 B-ULS, gold grill.  It has a random popping noise which seems to settle down a bit after being powered up for a half hour or so. 

If I enable the low frequency roll-off filter, the popping is greatly reduced, but still there.  This tells me the problem is most likely upstream from the filter.  The noise is worse in the cardioid position, which makes me think it's a problem with the front capsule.  Would you agree?

THX, GY

[radiovinhet]

Gold grill - it's a C414 Tl-II!

The problem can be the capsule. If the noise get more louder in cardioid mode, the front membrane is possible dirty, and will be more sensitive to moisture, that will make some noise.
You can: swap the capsule sides (front to back)... or try to clean the diafragm.

[klaus]

An easy and quite reliable method to determine whether a noise problem in a condenser mic originates at the capsule or at the mic's electronics is this:

If you can trigger, exacerbate, or otherwise influence the noise by accessing the capsule with your mouth up close through breathing or popping or yelling into it, it is a capsule charge issue. Most often a diaphragm contamination and or moisture issue.

If the characteristics and volume of the noise is unaffected by your adding SPLs, no matter how loudly you speak or yell or how moist you breathe into the capsule, the cause is electronic, and not the capsule.

[Jim Williams]

Clean the pattern selector switch. This is a common problem with those mics.

[GYMusic]

I appreciate the replies.  Yes, it is a Tl-II.  Yelling/breath up close did not make the noise any worse.  I have a link to a WAV file of the noise.  The recording level is set at a vocal recording level.  You might have to boost to the level.  I thought it would be more useful to post it at a typical level.  I will clean the switches, but I have rocked them quite a few times and that doesn't seem to help.  I'm suspecting a noisy cap or XTR.

There's a pretty nice "pop" @ 22 seconds into the clip.

http://www.gymusic.com/audio/AKG414_Noise.wav

[klaus]

That points to an electronic culprit.  Troubleshooting an 8 transistor mic (corrected by Jim Williams: it's actually 16, counting power supply and amp sections! K.H.), and many capacitors (which this discharge sound may point to can be a pain.

Speaking from experience, the tantalum filter caps are notorious for phantom spike damage/overvoltage intolerance. I would start substituting these before anything else.

[GYMusic]

Yes, I'll give it a go and start with the caps.  TNX

[Jim Williams]

There are 17 transistors in that circuit and a number of capacitors. The pcb's are very tiny and the tantalum caps small and very closely packed. They are mounted right up against the connectors so usually you need to remove those before the caps, in other words, these are a tiny PITA to de-solder successfully. You will need very narrow width desoldering tools to avoid damage to ajacent components and quite a bit of skill.

Rather than 'shotgun' out parts nilly-willy, use a scope and trace the signal and find exactly where this problem occurs. Otherwise there is a very good chance you will damage this mic with un-neccessary parts swaps.

If the head amp is the problem, usually it's not a transistor but one of the four 3 gig resistors that are soaked in conformal coating. Removing them usually means destroying them so better you have a set of spares, IF you find this is the section exhibiting the problems. The yellow WIMA polycarbonate capacitors used in the head amp are no longer manufactured and are not available. You will have to re-use those parts if they are not damaged or fit in some inferior MKS mylar caps. When I used to rebuild these I would fit in 470 pf WIMA FKP-2's at 1000 volt ratings, much better than the stock polycarbonate caps, but a tough fit.

You can also send it to Richard Land in Nashville if you have any doubts about your abilities and the availability of spare parts. He will swap the card.

[GYMusic]

I had an RF transmitter apart the other day doing some mods on an SMI board.  My hands are still pretty steady!  I've got a nice soldering pencil with some tiny tips and a magnifying lamp.  Years of experience, but age is making me question my confidence these days.  I'm thinking... I use canned freeze spray on audio amps to find the bad component in cramped spaces.  I usually use it in this same circumstance where the component is displaying intermittent noise.  Freezing it makes it act up.   

[GYMusic]

It's been a while, but I thought I would post my find.  I replaced the 330 uF @ 50 v electrolytics that were the problem.  Easy fix.

[klaus]

Thanks for following up! That really improves the quality of the thread and, by extension, that of the forum.

[FrederickThorne]

I wanted to add to this topic and microphone repair. For those who have also run into this problem with their AKG C414B-ULS (TLII)

I own an AKG C414B-ULS and I was also experiencing the described popping noises. The noises were present at random intervals and not affected by SPL.

The pops and clicks were radical when switching the Pad, or High Pass shelf and pattern sections. I was able to find the schematic PDF for the microphones; C414B-ULS / C414B-TL. They vary a little between the two models in that the C414B-ULS did not have the mentioned 330uf @ 50v capacitor. THE B-ULS had instead: 220uf @ 10v capacitor (same circuit).

http://recordinghacks.com/pdf/akg/C414B-TLII-service-doc.pdf

[klaus]

So what was the problem and its solution in your case?

[FrederickThorne]

So what was the problem and its solution in your case?

quick overview: I have the C414B-ULS and it has consistent popping/plosive noises at random intervals.

My plan was to identify and replace all of the polarized caps and see if that solves the problem.

I replaced the 330uf @ 50v capacitor first and it did not solve the random popping noise. I then replaced 4 of the 10u/25v (tantalum capacitors on the same circuit board as the 330uf @ 50v electrolytic cap) - I am still experiencing the random popping.

My next step is replace the one remaining 10u/25v tantalum cap on the opposite board.

So, that is basically where I am at this point.

Hopefully be back soon with good news.

Cheers